Sugar transport sequences, yeast strains having improved sugar uptake, and methods of use

ABSTRACT

Disclosed are nucleic acid constructs comprising coding sequences operably linked to a promoter not natively associated with the coding sequence. The coding sequences encode  Pichia stipitis  proteins that allow recombinant strains of  Saccharomyces cerevisiae  expressing the protein to grow on xylose, and allow or increase uptake of xylose by  Pichia stipitis  or  Saccharomyces cerevisiae  expressing the coding sequences. Expression of the coding sequences enhances uptake of xylose and/or glucose, allowing increased ethanol or xylitol production.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority to U.S. Provisional PatentApplication No. 61/061,417 filed Jun. 13, 2008, which is incorporated byreference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention is jointly owned between WARF and the USDA and was madewith United States government support awarded by the following agencies:

USDA Grant Number 2006-35504-17436.

The government has certain rights in this invention.

BACKGROUND OF THE INVENTION

Within the United States, ongoing res is directed toward developingalternative energy sources to reduce our dependence on foreign oil andnonrenewable energy. The use of ethanol as a fuel has becomeincreasingly prevalent in recent years. Currently, corn is the primarycarbon source used in ethanol production. The use of corn in ethanolproduction is not economically sustainable and, arguably, may result inincreased food costs.

In order to meet the increased demand for ethanol, it will be necessaryto ferment sugars from other biomass, such as agricultural wastes, cornhulls, corncobs, cellulosic materials, pulping wastes, fast-growinghardwood species, and the like. Biomass from most of these sourcescontains large amounts of xylose, constituting about 20 to 25% of thetotal dry weight. Because agricultural residues have a high xylosecontent, the potential economic and ecologic benefits of convertingxylose in these renewable materials to ethanol are significant.

In biomass conversion, microorganisms serve as biocatalysts to convertcellulosic materials into usable end products such as ethanol. Efficientbiomass conversion in large-scale industrial applications requires amicroorganism that can tolerate high sugar and ethanol concentrations,and which is able to ferment multiple sugars simultaneously.

The pentose D-xylose is significantly more difficult to ferment thanD-glucose. Bacteria can ferment pentoses to ethanol and otherco-products, and bacteria with improved ethanol production from pentosesugars have been genetically engineered. However, these bacteria aresensitive to low pH and high concentrations of ethanol, their use infermentations is associated with co-product formation, and the level ofethanol produced remains too low to make the use of these bacteria inlarge-scale ethanol production economically feasible.

In general, industrial producers of ethanol strongly favor using yeastas biocatalysts, because yeast fermentations are relatively resistant tocontamination, are relatively insensitive to low pH and ethanol, and areeasier to handle in large-scale processing. Many different yeast speciesuse xylose respiratively, but only a few species use xylosefermentatively. Fermentation of xylose to ethanol by wild typexylose-fermenting yeast species occurs slowly and results in low yields,relative to fermentation rates and ethanol yields that are obtained withconventional yeasts in glucose fermentations. In order to improve thecost effectiveness of xylose fermentation, it is necessary to increasethe rate of fermentation and the ethanol yields obtained.

The most commonly used yeast in industrial applications is Saccharomycescerevisiae. Although S. cerevisiae is unable to grow on or fermentxylose, homogenates of S. cerevisiae can readily fermentD-ribulose-5-phosphate to ethanol, and can convertD-xylulose-5-phosphate to a lesser extent. S. cerevisiae metabolicallyengineered to overproduce D-xylose reductase (XYL1), xylitoldehydrogenase (XYL2) and D-xylulokinase (XYL3 or XKS1) or some forms ofxylose isomerase (xylA) along with AYL3 or XKS1 can metabolize xylose toethanol.

Pichia stipitis is a yeast species that in its native state is able toferment xylose to produce ethanol. In P. stipitis, fermentative andrespirative metabolism co-exist to support cell growth and theconversion of sugar to ethanol.

There is a need in the art for yeast strains having improved ability toconvert sugars to ethanol.

BRIEF SUMMARY OF THE INVENTION

In one aspect, the present invention provides a nucleic acid constructcomprising a coding sequence operably linked to a promoter not nativelyassociated with the coding sequence. The coding sequence encodes apolypeptide having at least 95% amino acid identity to SEQ ID NO:2, SEQID NO:4, or SEQ ID NO:6.

Other aspects of the invention provide yeast strains comprising thenucleic acid construct, and methods of producing ethanol or xylitol bygrowing a yeast strain of the invention in xylose-containing medium.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 shows the alignment of PsSUT2, PsSUT3, and PsSUT4.

FIG. 2 shows growth, measured as OD₆₀₀, of S. cerevisiae expressing XUT1or SUT4 on xylose as a function of time.

FIG. 3 shows growth, measured as OD₆₀₀, of S. cerevisiae expressing XUT1or SUT4 on glucose as a function of time.

FIG. 4 shows uptake of ¹⁴C-glucose or ¹⁴C-xylose, measured as DPM, by S.cerevisiae expressing XUT1 or SUT4 as a function of time.

FIG. 5 shows consumption of glucose and xylose by Pichia stipitisoverexpressing SUT 4 as a function of time.

FIG. 6 shows glucose consumption and ethanol production by Pichiastipitis overexpressing SUT4 as a function of time.

DETAILED DESCRIPTION OF THE INVENTION

One approach to increasing the efficiency of yeast-catalyzed conversionof sugars to ethanol or to other products such as xylitol is to enhanceuptake of sugars by the yeast. Cellulosic and hemicellulosic materialsused in fermentation reactions are generally subjected to pretreatmentsand enzymatic processes that produce a mixture of several sugars,including glucose, xylose, mannose, galactose, and arabinose.

Saccharomyces cerevisiae and other hexose-fermenting yeasts have sugartransporters that facilitate uptake of hexoses. Some hexose sugartransporters also promote uptake of pentoses such as xylose, but at amuch lower rate and with much lower affinity. Therefore, xylose uptakeand utilization is impaired until glucose is consumed. When thesubstrate includes large initial concentrations of glucose, ethanolproduction from fermentation of glucose may result in inhibitoryconcentrations of ethanol before xylose uptake begins.

In order to promote xylose utilization by yeast in fermentation of mixedhemicellulosic sugars, sugar transport proteins having high affinity orspecificity for xylose were identified and used to develop yeastengineered to have improved xylose uptake, as described below in theExamples.

The genome of the xylose-fermenting yeast Pichia stipitis was examinedfor putative sugar transporters based on similarity to transporters thatfacilitate uptake of hexoses, lactose, maltose, phosphate, urea, andother compounds. Eight putative xylose transporters (XUT1-7 and SUT4)were chosen based on their implied structures and sequence similaritiesto other reported sugar transporters. Genome array expression studieswere used to evaluate whether the sugar transporters showed inducedexpression in cells grown on xylose. Genome array expression resultswere confirmed using quantitative PCR of transcripts from cells grown onxylose.

Sequences encoding polypeptides corresponding to those encoded by twogenes (XUT1 and SUT4) that showed increased expression in cells grown onxylose-containing media under oxygen limited conditions were expressedin a host strain Saccharomyces cerevisiae JYB3011, which lacks allhexose-transport genes and expresses P. stipitis XYL1, XYL2, and XYL3,which encode xylose reductase (XR), xylitol dehydrogenase (XDH), andxylulokinase (XK), respectively. Expression of either XUT1 or SUT4allowed growth of the S. cerevisiae strain on xylose. Both XUT1 and SUT4allowed increased xylose uptake. Cells carrying these sequences weretested for their relative affinities in taking up ¹⁴C-labeled glucose or¹⁴C-labeled xylose. Xut1 was shown to have a higher affinity for xylose,and Sut4 a higher affinity for glucose. A transformant carrying apartial sequence of the XUT1 gene, which encodes the C-terminal portionof the protein, beginning with amino acid residue 154 of SEQ ID NO:2,was shown to bind xylose and glucose but did not accumulate thesesugars, thereby demonstrating that at least some portion of the aminoterminus of the protein is needed for sugar transport.

Thus, expression of any of XUT1, XUT1 partial, and SUT4 in Saccharomycescerevisiae allows growth on xylose, and in the case of XUT1 and SUT4,uptake of xylose. It is envisioned that expression of any of XUT1, XUT1partial, and SUT4 or sequences similar to XUT1, XUT1 partial, and SUT4in other yeast species will also allow growth on xylose and/or uptake ofxylose.

The sequence of the deduced amino acid sequence of the putative proteinsencoded by the Pichia stipitis XUT1 and SUT4 genes are provided in SEQID NO:2 and SEQ ID NO:4, respectively. The sequences of the XUT1 andSUT4 coding sequences prepared from Pichia stipitis are shown in SEQ IDNO:1 and SEQ ID NO:3, respectively.

The P. stipitis glucose/xylose facilitators PsSut1-4 have a high degreeof sequence identity, with PsSut1 having 78% amino acid identity withPsSut2, 3, and 4. PsSut2 differs from PsSut3 by one amino acid (L279V).PsSut4 differs from PsSut2 by just one amino acid A544T) and from PsSUT3by two amino acids (L279V and A544T ). While all four facilitators havesimilar affinities for glucose, their affinities for xylose were shownto differ significantly (See Table 3). Notably, PsSut4 has a higherspecificity ratio (V_(max)/K_(m)) than the other Sut facilitators forglucose and xylose, and a higher affinity for xylose than any of theother facilitators.

It is envisioned that changes to the polypeptides could be made withoutaffecting their activities, and such variations are within the scope ofthe invention. For example, polypeptides have insertions, deletions orsubstitutions of amino acids relative to SEQ ID NO:2 and SEQ ID NO:4 arewithin the scope of the invention, provided that the polypeptide has atleast 80% amino acid identity to SEQ ID NO:2 and SEQ ID NO:4. Suitably,a polypeptide according to the invention has at least 85%, 90%, or 95%amino acid identity to SEQ ID NO:2 or SEQ ID NO:4. It is furtherenvisioned that polypeptides according to the invention have at least80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, or 100% amino acid identity toSEQ ID NO:4, and have a threonine residue or a serine residue at aposition corresponding to amino acid 544 of SEQ ID NO:4. Preferably,polypeptides according to the invention retain their activity, i.e.,retain the ability to enhance uptake of glucose and/or xylose. Suchactivity may be assayed by any suitable means, including, for example,those described in the examples.

It is well understood among those of ordinary skill in the art thatcertain changes in nucleic acid sequence make little or no difference tothe overall function of the protein or peptide encoded by the sequence.Due to the degeneracy of the genetic code, particularly in the thirdposition of codons, changes in the nucleic acid sequence may not resultin a different amino acid being specified by that codon. Changes thatresult in an amino acid substitution may have little or no effect on thethree dimensional structure or function of the encoded protein orpeptide. Conservative substitutions are even less likely to result in alost of function. In addition, changes that result in insertions ordeletions of amino acids may also be acceptable.

Alignment of the native sequence encoding PsSut4 with those encodingPsSut2 and PsSut3 shows that PsSut4 differs from the coding sequencesfor PsSut2 and PsSut3 at more than 30 bases (FIG. 1), with only twodifferences resulting in a change in the amino acid specified. Suitably,polynucleotides according to the invention include one or more of thedivergent sequences of PsSUT4 shown in FIG. 1, in any combination.Suitably, the polynucleotides include anywhere from one to all of thedivergent sequences of PsSUT4 shown in FIG. 1.

It is expected that expression of more than one of the proteins encodedby the polynucleotides of the invention, as exemplified by SEQ ID NO:2,SEQ ID NO:4, and SEQ ID NO:6, could be obtained in a transgenic yeastcell. Expression of two or more polynucleotides of the invention mayafford additional advantages in terms of fermentation rate or sugaruptake. For example, one could create a yeast strain that expresses apolynucleotide encoding a polypeptide having at least 80%, at least 85%,at least 90%, or at least 95% amino acid identity to SEQ ID NO:2together with a polynucleotide encoding a polypeptide having at least80%, at least 85%, at least 90%, or at least 95% amino acid identity toSEQ ID NO:4.

Additionally, the polynucleotides of the invention may further include asequence encoding a selectable marker that would allow selection ofyeast expressing the polynucleotide.

In the examples, expression of SUT4 under the control of the promoterfrom P. stipitis fatty acid synthase 2 (FAS2 promoter), which is inducedby xylose and under oxygen limiting conditions, was obtained in S.cerevisiae yJML123 and in P. stipitis UC7. The yJML123+SUT4 was able touse both glucose and xylose at a faster rate than the control strain.Neither strain was able to use all of the xylose. The yJML123+SUT4strain, which has reduced expression of xylitol reductase, produced ahigher yield of xylitol (18.45 g/L) than the control strain (15.51 g/L).Further, the yJML123+SUT4 strain produced a higher yield of xylitolfaster than the control.

The P. stipitis UC7+SUT4 strain was grown under oxygen limitingconditions (50 mL of culture with a starting OD₆₀₀ of 10 in a 125 mLflask aerated at speed of 100 rpm at 30° C.). The UC7+SUT4 strain wasfound to use xylose at a faster than the UC7 control; neither strainused all of the xylose. The xylitol yield of UC7+SUT4 strain (14.98 g/L)was similar to that of the UC7 control (15.18 g/L). The UC7+SUT4 strainhad a higher ethanol specific productivity.

In another example, UC7+SUT4 strain was grown in glucose (7% w/w) andxylose (3% w/w) in a three liter bioreactor. Those results indicatedthat the UC7+SUT4 grew at a faster rate than did the UC7 control. TheUC7+SUT4 strain had a higher biomass yield than the control.Additionally, the ethanol yield was higher for the UC7+SUT4 strain. TheUC7+SUT4 strain consumed both glucose and xylose at a higher rate andconsumed all of the sugars, whereas the control consumed only 88% of theglucose and did not appear to consume xylose. Finally, the specific rateof ethanol production on a dry cell weight basis was increased by morethan three fold.

In the examples, strains were developed by causing nucleic acid of theinvention to be introduced into a chromosome of the host yeast strain.This can be accomplished by any suitable means. However, it isenvisioned that one could obtain increased expression of the nucleicacid constructs of the invention using an extrachromosomal geneticelement, by integrating additional copies, by increasing promoterstrength, or by increasing the efficiency of translation through codonoptimization, all methods known to one of skill in the art.

It is envisioned that in addition to the particular strains that wereused, any strain of S. cerevisiae or P. stipitis could be used in thepractice of the invention. For example, in addition to P. stipitis UC7,one could advantageously obtain expression of the polynucleotides of theinvention in P. stipitis strains having reduced respiration capacity,such as petite mutants or P. stipitis Shi21 (NRRL Y-21971). Similarly,one could advantageously obtain expression of the polynucleotides of theinvention in a PHO-13 mutant of S. cerevisiae expressing P. stipitisXYL1, XYL2, and XYL3. Such a strain was developed, as described in theexamples, for use in developing yeast strains according to theinvention. Yeasts strains may suitably be modified to include exogenoussequences expressing Xut1 and Sut4 on the same or different nucleicacid. The yeast strains may be modified to include additionaladvantageous features, as would be appreciated by one skilled in theart.

In addition to S. cerevisiae and P. stepitis, it is envisioned thatother yeast species could be used to obtain yeast strains according tothe invention for use in the methods of the invention. Other suitableyeast species include, without limitation, Candida boidinii, Enteroramusdimorphus, Candida jeffriesii, Debaryomyces hansenii, CandidaGuillermondii, Candida shehatae, Brettanomyces naardensis, Candidaguillermondii, Candida lyxosophilia, Candida intermedia, Candida tenuis,Hansenula polymorpha, Kluyveromyces marxianus, Kluyveromyces lactis,Kluyveromyces fragilis, Kluyveromyces thermotolerans, Pachysolentannophilus, Pichia segobiensis, Spathaspora passalidarum and Pass 1isolates.

In another aspect, the present invention provides a method of fermentingxylose in a xylose-containing material to produce ethanol using theyeast of the invention as a biocatalyst. Another aspect of the presentinvention provides a method of fermenting xylose in a xylose-containingmaterial to produce xylitol using the yeast of the invention as abiocatalyst. In this embodiment, the yeast preferably has reducedxylitol dehydrogenase activity such that xylitol is accumulated.Preferably, the yeast is recovered after the xylose in the medium isfermented to ethanol and used in subsequent fermentations.

The constructs of the invention comprise a coding sequence operablyconnected to a promoter. Preferably, the promoter is a constitutivepromoter functional in yeast, or an inducible promoter that is inducedunder conditions favorable to uptake of sugars or to permitfermentation. Inducible promoters may include, for example, a promoterthat is enhanced in response to particular sugars, or in response tooxygen limited conditions, such as the FAS2 promoter used in theexamples. Examples of other suitable promoters include promotersassociated with genes encoding P. stipitis proteins which are induced inresponse to xylose under oxygen limiting conditions, including, but notlimited to, myo-inositol 2-dehydrogenase (MOR1), aminotransferase(YOD1), guanine deaminase (GAH1). These proteins correspond to proteinidentification numbers 64256, 35479, and 36448 on the Joint GenomeInstitute Pichia stipitis web site:genome.jgi-psf.org/Picst3/Picst3.home.html.

Oxygen limiting conditions include conditions that favor fermentation.Such conditions, which are neither strictly anaerobic nor fully aerobic,can be achieved, for example, by growing liquid cultures with reducedaeration, i.e., by reducing shaking, by increasing the ratio of theculture volume to flask volume, by inoculating a culture medium with anumber of yeast effective to provide a sufficiently concentrated initialculture to reduce oxygen availability, e.g., to provide an initial celldensity of 1.0 g/l dry wt of cells.

By “xylose-containing material,” it is meant any medium comprisingxylose, whether liquid or solid. Suitable xylose-containing materialsinclude, but are not limited to, hydrolysates of polysaccharide orlignocellulosic biomass such as corn hulls, wood, paper, agriculturalby-products, and the like.

By a “hydrolysate” as used herein, it is meant a polysaccharide that hasbeen depolymerized through the addition of water to form mono andoligosaccharides. Hydrolysates may be produced by enzymatic or acidhydrolysis of the polysaccharide-containing material, by a combinationof enzymatic and acid hydrolysis, or by an other suitable means.

Preferably, the yeast strain is able to grow under conditions similar tothose found in industrial sources of xylose. The method of the presentinvention would be most economical when the xylose-containing materialcan be inoculated with the mutant yeast without excessive manipulation.By way of example, the pulping industry generates large amounts ofcellulosic waste. Saccharification of the cellulose by acid hydrolysisyields hexoses and pentoses that can be used in fermentation reactions.However, the hydrolysate or sulfite liquor contains high concentrationsof sulfite and phenolic inhibitors naturally present in the wood whichinhibit or prevent the growth of most organisms. Serially subculturingyeast selects for strains that are better able to grow in the presenceof sulfite or phenolic inhibitors.

It is expected that yeast strains of the present invention may befurther manipulated to achieve other desirable characteristics, or evenhigher specific ethanol yields. For example, selection of mutant yeaststrains by serially cultivating the mutant yeast strains of the presentinvention on medium containing hydrolysate may result in improved yeastwith enhanced fermentation rates.

The following non limiting examples are intended to be purelyillustrative.

EXAMPLES

Characterization of Putative Xylose Transporters of Pichia stipitis

Yeast strains, plasmids, and growth conditions. The yeast strains andthe plasmids used in this study are listed in Table 1. S. cerevisiaeEBY.VW4000 strain (MATa Δhxt1-17 Δgal2 Δstl1 Δagt1 Δmph2 Δmph3leu2-3,112 ura3-52 trp1-289 his3-Δ1 MAL2-8c SUC2) (9) was kindlyprovided by E. Boles at Heinrich-Heine-Universitat and was used fortransformation of all the constructed plasmids. Three xylose transportercandidates, XUT1, XUT1 partial (corresponding to the sequence encodingthe C-terminal region of the protein beginning with amino acid residue154 of SEQ ID NO:2), and SUT4, were amplified and fused with the TDH3promoter by PCR. The coding sequence of SUT4 includes a CUG codon, whichin P. stipitis, specifies the serine residue of amino residue 410 of SEQID NO:4. This codon, which in most organisms specifies a leucine, wasmodified for expression in S. cerevisiae using standard methods so as tospecify a serine residue. Each candidate had its own terminator. Thesefragments were then inserted into pRS316 after sequence confirmation(Sikorski & Hieter (1989) Genetics 122, 19-27). Genetic manipulation andcloning of DNA were performed using methods known in the art.Escherichia coli DH5αF′ was used as a host for plasmid preparation.

Yeast transformation was performed as previously described (Gietz &Woods (2002) Methods Enzymol 350, 87-96). Yeast transformants wereselected by TRP1 and URA3 selectable markers and cultivated on yeastsynthetic complete dropout (TRP− URA−) medium with carbon source (Kaiseret al. & Cold Spring Harbor Laboratory. (1994) Methods in yeastgenetics: a Cold Spring Harbor Laboratory course manual (Cold SpringHarbor Laboratory Press, Cold Spring Harbor, N.Y.)). Yeast cells werecultured in a 125-ml flask with 50 ml medium at 30° C. and 200 rpm.

Quantitative PCR (qPCR) Total RNA was prepared from each transformantand 5 μg of total RNA was used for cDNA construction with randomoligonucleotides and the Reverse Transcription System Kit (Promega,Madison, Wis.). Reverse Transcription-PCR primers were designed usingPrimer Express software (Applied Biosystems). RT-PCR was performed withSYBR green PCR master mix (Applied Biosystems) and an ABI PRISM 7000sequence detection system (Applied Biosystems). PCR conditions were asrecommended by the manufacturer except that one-half of the reactionvolume was used. Actin was used to normalize the relative copy numbersof each gene. All reactions were performed in triplicate.

TABLE 1 List of yeast strains and plasmids Source or Strain or plasmidDescription reference Strains S. cerevisiae EBY.VW4000 MATa Δhxt1-17Δgal2 Δstl1 Δagt1 Δmph2 Wieczorke Δmph3 leu2-3,112 ura3-52 trp1-289his3-Δ1 MAL2-8^(c) SUC2 S. cerevisiae JYB3005 S. cerevisiaeEBY.VW4000(pRS314 and This study pRS316) S. cerevisiae JYB3011 S.cerevisiae EBY.VW4000(pRS314-X123 and This study pRS316) S. cerevisiaeJYB3110 S. cerevisiae EBY.VW4000(pRS314-X123 and This studypRS316-XUT1partial) S. cerevisiae JYB3210 S. cerevisiaeEBY.VW4000(pRS314-X123 and This study pRS316-XUT1) S. cerevisiae JYB3310S. cerevisiae EBY.VW4000(pRS314-X123 and This study pRS316-SUT4)Plasmids pRS314 TRP1 CEN/ARS Sikorski pRS316 URA3 CEN/ARS SikorskipRS314-X123 TRP1 CEN/ARS TDH3_(p-)PsXYL1-TDH3_(T) NiTDH3_(p-)PsXYL2-TDH3_(T) TDH3_(P)-PsXYL3-TDH3_(T) pRS316-XUT1partialURA3 CEN/ARS TDH3_(P)-PsXUT1 This study pRS316-XUT1 URA3 CEN/ARSTDH3_(P)-PsXUT1 This study pRS316-SUT4 URA3 CEN/ARS TDH3_(P)-PsSUT4 Thisstudy

Sugar-transport assay. Initial D-[14C]xylose and D-[¹⁴C]glucose(Amersham) uptake rates were measured at 30° C., pH 5.0 as previouslydescribed (Spencer-Martins, I. a. c. U., N. (1985) Biochim. Biophys.Acta 812, 168-172). Cells were harvested in the mid-exponential phase(A₆₀₀nm 0.8-1.0; ≈0.1-0.125 g/l dry wt) by centrifugation, washed twicewith ice-cold water and resuspended to a cell concentration of 5-10 mg(dry weight)/ml. Kinetic parameters were determined usingLineweaver-Burk plots. All measurements were conducted at least threetimes, and the data reported are the average values.

Transcriptional analysis of eight xylose transporter candidates. Eightputative xylose transporters (XUT1-7 and SUT4) were identified based ongenomic analysis of the genome sequence of Pichia stipitis (Jeffries etal. (2007) Nat Biotechnol 25, 319-26) and selected for further analysis.Of these eight putative transporters, XUT1, and SUT4 showed increasedexpression in Pichia stipitis grown in xylose-containing media. Thesetwo genes were introduced into the host strain S. cerevisiae JYB3011 toevaluate the ability of the putative xylose transporters to promotexylose uptake. This host strain was originated from S. cerevisiaeEBY.VW4000 (Wieczorke et al. (1999) FEBS Lett 464, 123-8) and engineeredto express PsXYL1, PsXYL2, and PsXYL3 by introducing pRS314-X123 (Ni etal. (2007) Appl Environ Microbiol 73, 2061-6) (Table 1).

Kinetic characteristics of sugar uptake. XUT1 partial, XUT1 and SUT4were expressed in the strains S. cerevisiae JYB3110, JYB 3210, andJYB3310, respectively (Table 1). The expression of each transporter inits respective strain was confirmed by qPCR (data not shown). Thesethree strains could grow in xylose-containing media (FIG. 2) and inglucose-containing media (FIG. 3). Xylose and glucose transport capacityof each candidate was tested using a high concentration (50 mM) oflabeled sugar (Table 2). The strains containing Xut1 or Sut4 showedincreased xylose uptake activities, with the strain expressing Xut1having a higher uptake rate than that of the strain expressing Sut4(FIG. 4). Both JYB 3210 and JYB3310 showed glucose-transportingactivity, with Sut4 conferring a higher glucose uptake velocity thanXut1 (FIG. 4).

Kinetic analysis demonstrated that the affinity of Xut1 for xylose is3-fold higher than its affinity for glucose (FIG. 4 and Table 3). Thisis the first reported transporter having a higher affinity for xylose.Vmax was also higher with xylose than glucose. Although Sut4 exhibitsxylose uptake activity, its affinity for glucose is 12 times higher thanits affinity for xylose (Table 3). Interestingly, Sut4 has higher Vmaxwith xylose than glucose. Notably, Sut4 has a higher specificity ratio(Vmax/Km) for both xylose and glucose than any of the other fourglucose/xylose facilitators found in Pichia stipitis (Table 3).

Table 3 compares Km values of the newly discovered xylose transportersto previously characterized xylose transporters. All four of the PsSutglucose/xylose facilitators show similar affinities for glucose, buttheir affinities for xylose differ significantly. Notably, PsSut4 showsa higher specificity ratio (Vmax/Km) for glucose and xylose than thespecificity ratios of PsSut1-3. Interestingly, PsSut1 has about 78%amino acid identity with PsSut2, 3 and 4, whereas PsSut2 and PsSut4differ from PsSut3 by one or two amino acids each (Sut2: L279V; Sut4:L279V, A544T). Despite the high degree of sequence identity, PsSut1-4have distinct properties. PsSut1 and PsSut3 can mediate fructosetransport, whereas PsSut2 cannot, and only PsSut3 can mediate galactosetransport (Weierstall et al. (1999) Molecular Microbiol. 31:871-883).Notably, among PsSut1-4, PsSut4 has the highest affinity and the secondhighest Vmax for xylose. The affinity of PsSut4 for xylose is aboutthree times higher than that of PsSut2, and its Vmax for xylose uptakeis about 4.4 times higher than the Vmax of Sut2.

Energy requirements for sugar transport were evaluated by including themetabolic uncoupler sodium azide. Xut1 mediated uptake of sugar wasreduced by 96% in the presence of sodium azide, whereas Sut4 mediatedsugar uptake was reduced by about 50% (Table 4). Consequently, Xut1 isbelieved to be a high affinity xylose-proton symporter, and SUT4 appearsto employ facilitated diffusion.

TABLE 2 Initial uptake rates of labeled sugar (50 mM) of each putativexylose transporter. Strain Substrate V^(‡) JYB3011 (Control) Glucose 5Xylose 0 JYB3110 (XUT1 Glucose 7 partial) Xylose 0 JYB3210 (XUT1)Glucose 31.8 Xylose 57.8 JYB3310 (SUT4) Glucose 52.6 Xylose 23.1^(‡)Velocity of uptake (milimoles per minutes per milligram [dryweight])

TABLE 3 Kinetic parameters of yeast glucose/xylose transporters K_(m)V_(max) Specificity (mM) (nmol min⁻¹ mg dw⁻¹) V_(max)/K_(m) TransportersGlucose Xylose Glucose Xylose Glucose Xylose Reference PsSut1 1.5 ± 0.1145.0 ± 1.0   45 ± 1.0 132 ± 1.0  30.0 0.9 (4) PsSut3 0.8 ± 0.1 103 ±3.0  3.7 ± 0.1  87 ± 2.0 4.6 0.8 (4) PsSut2 1.1 ± 0.1 49.0 ± 1.0  3.3 ±0.1  28 ± 4.0 3.3 0.6 (4) PsSut4 1.3 ± 0.1 16.6 ± 0.3  105 ± 4.2  122 ±2.4  80.8 7.4 This study CiGxf1 2.0 ± 0.6 48.7 ± 6.5  ≈163^(†) ≈25^(†)81.5 0.5 (1) ScHxt1 ^(a)107 ± 49   ^(b)880 ± 8   ^(a)50.9 ± 3.7  ^(b)750 ± 94   0.5 0.8 ^(a)(2) ^(b)(3) ScHxt2 ^(a)2.9 ± 0.3  ^(b)260 ±130  ^(a)15.6 ± 0.9   ^(b)340 ± 10   5.4 1.3 ^(a)(2) ^(b)(3) ScHxt4^(a)6.2 ± 0.5  ^(b)170 ± 120  ^(a)12.0 ± 0.9   ^(b)190 ± 23   1.9 1.1^(a)(2) ^(b)(3) ScHxt7 ^(a)1.3 ± 0.3  ^(b)130 ± 9   ^(a)11.7 ± 0.3  ^(b)110 ± 7   90.0 0.8 ^(a)(2) ^(b)(3) CiGxs1 0.012 ± 0.004 0.4 ± 0.14.3 ± .33 6.5 ± 1.5 358.3 16.2 (1) PsXut1 0.91 ± 0.01 0.46 ± 0.02  80 ±1.0 116 ± 5.8  87.91 252.2 This study ^(†)Calculated from FIG. 2 of(1) 1. Leandro, M. J., P. GonÁalves, and I. Spencer-Martins. 2006. Twoglucose/xylose transporter genes from the yeast Candida intermedia:first molecular characterization of a yeast xylose-H+ symporter. TheBiochemical journal 395: 543-549. 2. Maier, A., B. Volker, E. Boles, andG. F. Fuhrmann. 2002. Characterisation of glucose transport inSaccharomyces cerevisiae with plasma membrane vesicles(countertransport) and intact cells (initial uptake) with single Hxt1,Hxt2, Hxt3, Hxt4, Hxt6, Hxt7 or Gal2 transporters. Fems Yeast Research2: 539-550. 3. Saloheimo, A., J. Rauta, O. V. Stasyk, A. A. Sibirny, M.Penttila, and L. Ruohonen. 2007. Xylose transport studies withxylose-utilizing Saccharomyces cerevisiae strains expressingheterologous and homologous permeases. Applied Microbiology AndBiotechnology 74: 1041-1052. 4. Weierstall, T., C. P. Hollenberg, and E.Boles. 1999. Cloning and characterization of three genes (SUT1-3)encoding glucose transporters of the yeast Pichia stipitis. MolecularMicrobiology 31: 871-883.

TABLE 4 Effect of sodium azide on glucose and xylose uptake^(‡). GlucoseXylose Control NaN₃ (50 mM) Control NaN₃ (50 mM) JYB3110 (SUT4) 41 0.3290 3.19 JYB3310 (XUT1) 40 25 13.4 9.2 ^(‡)Velocity of uptake (nmol min⁻¹mg⁻¹ [dry weight])Effect of Expression of Sut4 in P. stipitis

Construction of plasmids and transformation of yeast strains. A plasmidwas constructed to contain the P. stipitis coding sequence for the SUT4putative transporter under the control of the P. stipitis fatty acidsynthase subunit alpha (Fas2) promoter (FAS2p_SUT4_SUT4t_LoxP_URA3_LoxPplasmid). Roughly 100 ug of the FAS2p_SUT4_SUT4t_LoxP_Ura3_LoxP plasmidwas linearized using Hind III and ApaI, ethanol precipitated andresuspended in water to create a fragment that would integrate directlyinto the Pichia genome. The digested construct was then used in a LiAcprotocol and transformed into either the yJML123 (Gietz & Woods (2002)Methods Enzymol 350, 87-96)) or UC7 Pichia stipitis strains. The yJML123strain has a deletion in the XYL2 gene, which encodes the enzyme thatcatalyzes the conversion of xylitol to xylulose, allowing us to createPichia strains that will begin to accumulate xylitol.

Transformants were selected for via growth on ScD −Ura plates, whichcontain 0.62 g/L CSM −Leu −Trp −Ura (Bio 101 Systems) and dextrose (2%).Colonies were then picked and grown overnight in ScD −Ura liquid media.Genomic DNA was then extracted and evaluated by PCR to confirmintegration of the construct. As a control for these strains, a plasmidcontaining only the LoxP_Ura3_LoxP cassette was also integrated into theyJML123 and UC7 strains (yJML123 and UC7 controls).

Fermentation of yJML123+SUT4 Starter cultures were established byinoculating a single colony of yJML123+SUT4 strain or yJML123 controlfrom recently streaked plate into 25 mL fermentation media and grownovernight. The following morning, triplicate flask cultures of eachtransformant were started at an OD₆₀₀ of 0.5 (≈0.06 g/l dry wt ofcells). The transformants were confirmed using PCR genotyping. Thefermentation was run under aerobic conditions using an agitation speedof 200 rpm at 30° C. in 25 mL of media in a 125 mL flask in fermentationmedium (Yeast Nitrogen Base (1.7 g/L), urea (2.4 g/L), peptone (3.6g/L), with xylose (40 g/L) and glucose (17 g/L) as carbon sources.Samples were collected for analysis roughly every 8 hours for the first48 hours and then once every 24 hours.

The yJML123+SUT4 strain was able to use both glucose and xylose at afaster rate than the control strain. Neither strain was able to use allof the xylose. The yJML123+SUT4 strain produced a higher yield ofxylitol (18.45 g/L) than the control strain (15.51 g/L). Further, theyJML123+SUT4 strain produced a higher yield of xylitol faster than thecontrol.

Fermentation of UC7+SUT4. Starter cultures were established byinoculating a single colony of SUT4+UC7 strain or the UC7 control fromrecently streaked plates into 50 mL of fermentation media and grownovernight. The following morning, triplicate flasks of each transformantwere started at an OD₆₀₀ of 0.5 (≈0.06 g/l dry wt of cells). Thetransformants were confirmed using PCR genotyping. The fermentation wasrun in fermentation medium (Yeast Nitrogen Base (1.7 g/L), urea (2.4g/L), peptone (3.6 g/L), and xylose (40 g/L), under low oxygenconditions using an agitation speed of 100 rpm at 30° C. in 50 mL ofmedia in a 125 mL flask. The starting OD₆₀₀ was 10 (≈1.25 g/l dry wt ofcells). Samples were collected for analysis roughly every 12 hours forthe first 60 hours and then once every 24 hours.

Fermentation of UC7+SUT4. The UC7+SUT4 strain was found to use xylose ata faster rate than the UC7 control; neither strain used all of thexylose. The xylitol yield of UC7+SUT4 strain was similar to that of theUC7 control. The UC7+SUT4 strain had a higher ethanol specificproductivity than that of the UC7 control.

Evaluation of Expression of SUT4 in P. stipitis in Bioreactor

Strain Descriptions. The UC7+SUT4 strain described above (“SUT4”) andthe UC7+URA3 cassette strain (“control”) described above were evaluatedin a three liter bioreactor scale-up as described below.

3 L Bioreactor Scale-up of the SUT4 Transporter Overexpression Strain.Low oxygen bioreactor trials were used to compare two strains in alarger scale and under more controlled conditions than shake flasktrials.

Bioreactor Cultivations. A defined minimal medium containing trace metalelements and vitamins was used in all bioreactor cultivations. Thismedium was modified based on that described by Verduyn et al. (Verduynet al. (1992) Yeast 8:501-517) to include urea as a nitrogen source. Ithad the following composition: 2.4 g urea L⁻¹; 3 g KH₂PO₄ L⁻¹, 0.5 gMgSO₄ 7H₂O L⁻¹; 1 ml trace element solution L⁻¹; 1 ml vitamin solutionL⁻¹; and 0.05 ml antifoam 289 (Sigma A-8436) L⁻¹ (Jeffries & Jin (2000)Adv Appl Microbiol 47, 221-68). For these cultivations, a startingconcentration of 70 g L⁻¹ glucose and 30 g L⁻¹ xylose was used.

Cultivations were performed in 3 L New Brunswick Scientific BioFlo 110Bioreactors with working volumes of 2 liters. All cultivations wereperformed at 30° C. Agitation was set at 500 rpm and the pH wascontrolled at pH 5.0 by the addition of 5 N KOH and 5 NH₂SO₄. Aerationwas controlled at a rate of 0.5 vvm which corresponded to a rate of 1 Lmin⁻¹. Input gas was mixed using a gas proportioner to include 90% purenitrogen and 10% air, for a final oxygen concentration of approximately2%. Bioreactors were inoculated to an OD of approximately 1 and theirprogress followed for 74 hours.

During cultivation, the SUT4 strain appeared to grow faster than thecontrol. Calculated biomass rates and yields demonstrated that SUT4 hada slightly improved growth rate (˜1.2 fold) (0.062 vs. 0.0514 h⁻¹) andbiomass yield (0.151 g dry cell weight/g glucose vs. 0.145 g dry cellweight/g glucose, or 0.184 Cmmol vs. 0.177 Cmmol). As was observedpreviously in the shake flask experiments described above, the overalldifference in ethanol yield from glucose was not large. The SUT4 strainhad a slightly higher (˜1.11 fold) ethanol yield on glucose (0.372 gethanol/g glucose or 0.485 Cmmol) than the control strain (0.335 gethanol/g glucose or 0.437 Cmmol/Cmmol glucose).

The major difference between these two strains is in their kinetic ratesof glucose and xylose consumption. The SUT4 strain consumes glucose at amuch higher rate than the control strain (FIG. 5). In fact, the SUT4strain consumed all of the glucose and xylose in the fermentation, whilethe URA3 control strain consumed approximately 88% of the glucose anddid not consume enough xylose to significantly change the xyloseconcentration during the length of the fermentation.

The specific rate of glucose consumption for the SUT4 strain (0.284 gglucose gDCW⁻¹h⁻¹ or 9.448 Cmmol gDCW⁻¹h⁻¹) was 3.04 fold higher thanthat of the URA3 control strain (0.093 g Glucose gDCW⁻¹h⁻¹ or 3.106Cmmol gDCW⁻¹h⁻¹). The specific rates of xylose consumption could not becompared due to the fact that the URA3 control strain had notsignificantly begun to utilize the xylose. Further bioreactor trialswill be necessary to compare these rates.

The SUT4 and control strains also exhibited different rates of ethanolproduction. FIG. 6. shows a plot of ethanol production and glucoseconsumption. It should be noted that a small amount of ethanol producedby the SUT4 overexpression strain was due to the xylose consumed;however the rates and yields were calculated using the glucose onlyphase of fermentation. The specific rate of ethanol production for theSUT4 overexpression strain (0.106 g ethanol gDCW⁻¹h⁻¹ or 4.580 CmmolgDCW⁻¹h⁻¹) was 3.38 fold higher than that of the control strain (0.031 gETOH gDCW⁻¹h⁻¹ or 1.356 Cmmol gDCW⁻¹h⁻¹).

Creation of the Δpho13 Background Strains for Transporter Testing

In order to test the effect of putative transporters on xyloseutilization, a robust Saccharomyces cerevisiae background that iscapable of utilizing xylose in some capacity needed to be built.Previous research has shown that either the overexpression of TALI orthe deletion of PHO13 promotes growth on xylose when the Pichia stipitisxylulokinase gene is expressed at a high level (10). The PHO13 deletionwas chosen for use in these strains, as more detailed kinetic dataduring aerobic and low oxygen cultivations is available for a straincreated in a similar genetic background (Van Vleet et al. (2008) Metab.Eng. Doi10.1016/j.ymben.2007.12.002).

The strains CEN.PK.111-27B and CEN.PK.102-3A were chosen as the geneticbackgrounds in which to create these deletion strains (Entian K, KotterP, (2007) 25 Yeast Genetic Strain and Plasmid Collections. In: Methodsin Microbiology; Yeast Gene Analysis—Second Edition, Vol. Volume 36 (IanStansfield and Michael J R Stark ed), pp 629-666. Academic Press). TheCEN.PK.111-27B carries the trp1 and leu2 mutations and the CEN.PK.102-3Astrain carries the ura3 and leu2 mutations. The deletion of the PHO13 inthese backgrounds will allow for the putative transporters to be testedin the presence of the xylose utilization genes expressed moderatelyand, separately, at a high level. These transporters will also be ableto be tested at 2 different levels of expression. Moderate expression ofthe transporters will be possible using a centromeric LEU2 plasmid andhigh levels of expression will be possible using a multicopy LEU2plasmid with a 2 μm origin of replication.

The PHO13 deletion cassette was amplified out of the genomic DNA of S.cerevisiae CMB.JHV.pho13a (Van Vleet et al. (2008) Metab. Eng.Doi10.1016/j.ymben.2007.12.002) using the high fidelity Pfusionpolymerase (NEB, Finnzymes) and primers designed 400 bp upstream anddownstream of the PHO13 open reading frame. This cassette contains theKANMX gene and has been shown previously to confer G418 resistance toPHO13 knockout strains in S. cerevisiae (Van Vleet et al. (2008) Metab.Eng. Doi10.1016/j.ymben.2007.12.002).

HpaI sites were added onto the ends of this amplified fragment for easyexcision from a storage vector. The primers used in strain creation andconfirmation are listed in Table 5. The amplified fragment was ligatedinto the pCR4Blunt-TOPO vector and then transformed into TOP10 E. coliby a heat shock method. Transformants were selected for on kanamycinplates and digest checked to confirm the size of the insert. Afterverification, 100 μg of plasmid DNA was prepared using a Qiagen maxiprepand digested with HpaI to release the deletion cassette. The deletioncassette was then gel purified.

The purified deletion cassette was then transformed into the CEN.PKstrains using a standard lithium acetate method (Gietz & Woods (2002)Methods Enzymol 350, 87-96)). Outgrowth was performed after thetransformation for 3 hours in YPD liquid media, and then transformantswere selected for on YPD plates containing 250 mg/ml G418. After 2 daysof growth, growth of putative knockouts was confirmed by re-streaking onfresh G418 plates. These strains were then genotyped using primers thatwere designed 600 bp upstream and downstream of the PHO13 open readingframe. These primers amplify a product in both deletion and non deletionstrains, however in deletion strains the product is approximately 650 bplarger than in the non deletion strain. Multiple transformants for eachstrain were identified as containing the deletion. Transformantscontaining the deletion originating from CEN.PK.111-27B were designatedFPL.JHV.002 and those originating from CEN.PK.102-3A were designatedFPL.JHV.003.

The xylose utilization genes (XYL1, XYL2, and XYL3) under the control ofthe strong constitutive TDH3 promoter were then introduced to thestrains via plasmid. FPL.JHV.002 was transformed with the centromericvector pRS314-X123 (10) and designated FPL.JHV.004. At the same time,FPL.JHV.003 was transformed with the multicopy vector pYES2-X123 (10)and designated FPL.JHV.005. Both of the vector bearing strains carry aleu2 marker to allow for testing of the putative transporters. Theplasmids and yeast strains are described in Table 6.

TABLE 5 Primers Used Primer Name Function Sequence oFPL.JHV.021 PHO13F+ HpaI GATTACATGTTAACGATTGTTCGACGCAACTACCC (SEQ ID NO: 8) oFPL.JHV.022PHO13R + HpaI GATTACATGTTAACCTTCCCCAACAAGACCGAATTG (SEQ ID NO: 9)oFPL.JHV.023 PHO13 TTAAAACAAGAATTTGGGGAG Confirmation F (SEQ ID NO: 10)oFPL.JHV.024 PHO13 AAAGGTCTAATTATTCAATTTATCGAC Confirmation (SEQ ID NO:11)

Assessment of putative transporters by whole genome expression arraytechnology. Cells of Pichia stipitis were cultivated in a bioreactorwith either xylose or glucose as carbon sources along with mineralnutrients and urea for a nitrogen source using the medium of Verduyn etal. (16). Cells were grown either under fully aerobic or oxygen limitedconditions. Cells were harvested, their mRNA was extracted and analyzedusing a whole-genome expression array (NimbleGen). Transcriptscorresponding to putative sugar transporters were identified andannotated. (Results not shown). Transcripts for seven genes (HXT2.4,HXT4, SUC1.5, XUT1, SUT4, XUT7 and YBR2) were induced to a higher levelin cells grown on xylose than in cells grown on glucose. All but SUT4were induced to their highest levels on xylose under oxygen limitingconditions. Transcripts for seven genes (A UT1, HGT2, HUT1, AML4, RGT2,STL1 and SUT1) were all induced to their highest levels when cells werecultivated on glucose. Transcript for one gene (SUT1) was induced to itshighest level under oxygen limitation; the other six were induced totheir highest levels under aerobic conditions. The specific induction oftranscripts on xylose under aerobic or oxygen limited conditions wastaken as an indicator of the effective function of this gene when cellswere cultivated on xylose. Of these genes, SUT1, SUT2 and SUT3 werepreviously recognized and described as glucose/xylose facilitatortransport proteins.

It is to be understood that the invention is not limited in itsapplication to the details of construction and the arrangement ofcomponents set forth in the following description. The invention iscapable of other embodiments and of being practiced or of being carriedout in various ways. Also, it is to be understood that the phraseologyand terminology used herein is for the purpose of description and shouldnot be regarded as limiting. The use of “including,” “comprising,” or“having” and variations thereof herein is meant to encompass the itemslisted thereafter and equivalents thereof as well as additional items.

It also is understood that any numerical range recited herein includesall values from the lower value to the upper value. For example, if aconcentration range is stated as 1% to 50%, it is intended that valuessuch as 2% to 40%, 10% to 30%, or 1% to 3%, etc., are expresslyenumerated in this specification. These are only examples of what isspecifically intended, and all possible combinations of numerical valuesbetween and including the lowest value and the highest value enumeratedare to be considered to be expressly stated in this application.

1. A nucleic acid construct comprising a coding sequence operably linkedto a promoter not natively associated with the coding sequence, thecoding sequence encoding a glucose/xylose transporter polypeptide havingat least 95% amino acid identity to SEQ ID NO:4, wherein the polypeptidecomprises a serine residue at the amino acid position corresponding toposition 544 of SEQ ID NO:4.
 2. The construct of claim 1, wherein thepromoter is a constitutive promoter.
 3. The construct of claim 1,wherein the promoter is inducible under oxygen limiting conditions. 4.The construct of claim 1, wherein the polypeptide comprises SEQ ID NO:4.5. The construct of claim 1, wherein the coding sequence comprises atleast one of the sequences shown in FIG.
 1. 6. The construct of claim 1,wherein the coding sequence comprises SEQ ID NO:3.
 7. A vectorcomprising the construct of claim
 1. 8. A yeast strain comprising theconstruct of claim
 1. 9. The strain of claim 8, wherein the strainexhibits increased uptake of xylose relative to a control yeast lackingthe construct.
 10. The strain of claim 8, wherein the strain has ahigher specific rate of ethanol production relative to a control yeastlacking the construct.
 11. The strain of claim 8, wherein the strainexhibits increased xylitol production relative to a control yeastlacking the construct.
 12. The strain of claim 11 wherein the strain hasreduced xylitol dehydrogenase activity such that xylitol accumulates.13. The strain of claim 8, wherein the yeast strain is a Pichiastipitis.
 14. The strain of claim 8, wherein the yeast strain is aSaccharomyces cerevisiae.
 15. A method of producing ethanol from thefermentation of xylose comprising: culturing the yeast strain of claim 8in xylose-containing material under suitable conditions for a period oftime sufficient to allow fermentation of at least a portion of thexylose to ethanol.
 16. The method of claim 15, wherein thexylose-containing material further comprises glucose.
 17. A method ofproducing xylitol from the fermentation of xylose comprising the stepof: culturing the yeast strain of claim 9 in xylose-containing materialunder suitable conditions for a period of time sufficient to allowfermentation of at least a portion of the xylose to xylitol.
 18. Theconstruct of claim 2, wherein the constitutive promoter is TDH3.
 19. Theconstruct of claim 3, wherein the promoter is selected from the groupconsisting of FAS2, MOR1, YOD1 and GAH1.